• Clinical science



Legionellosis is an infection caused by Legionella pneumophila, a gram-negative rod that thrives in warm aqueous environments such as drinking-water systems, hot tubs, and air-conditioning units. Transmission occurs by inhaling contaminated, aerosolized water droplets. Legionellosis is a common nosocomial infection and outbreaks are typical. Notable risk factors include smoking, chronic lung disease, advanced age, and immunosuppressive conditions. There are two forms of legionellosis: Legionnaires' disease and Pontiac fever. Patients with Legionnaires' disease present with atypical pneumonia (shortness of breath, cough), commonly in combination with various other symptoms, including gastrointestinal (e.g., diarrhea) and neurological (e.g., confusion). Laboratory abnormalities are common, especially hyponatremia. Pontiac fever is a milder, self-limiting, flu-like illness. A urine antigen test is used to confirm infection with L. pneumophila. Fluoroquinolones are the treatment of choice for Legionnaires' disease. In the U.S., legionellosis is a notifiable disease and steps should be taken to eliminate contaminated sources and to prevent future outbreaks.


  • Frequency: occurs rarely in infants, almost solely in adults (of any age) and typically in outbreaks
  • High-risk patients


Epidemiological data refers to the US, unless otherwise specified.


Causative organism

Path of infection

  • Inhalation of contaminated aerosols
    • Cold and hot water systems: e.g., those found in hotels, hospitals, and retirement homes
    • Whirlpools/hot tubs, swimming pools, showers
    • Air-conditioning systems with contaminated condensed water

Common associations

  • Nursing homes
  • Hospitals
  • Confined travel accommodations (e.g., cruise ships, hotels, resorts)


Clinical features

Legionnaires' disease

In a patient with signs of atypical pneumonia and diarrhea in combination with possible exposure (e.g., cruise ship travel, use of a whirlpool), legionellosis must always be considered!

Pontiac fever

  • Incubation period: 1–3 days
  • Mild, self-limiting course of legionellosis without pneumonia
  • Symptoms are difficult to distinguish from the flu and include fever, headache, and muscle aches



See pneumonia diagnostics.

Laboratory findings

  • Blood
  • Urine
    • Urinary antigen test: the most important diagnostic tool
      • Rapid test, but only detects serogroup 1
    • Hematuria and proteinuria are common
  • Respiratory secretions
    • Gram stain of respiratory secretions shows many neutrophils; usually no organisms identified
      • Requires silver stain
    • Legionella culture: slow; requires buffered (iron and cysteine) charcoal yeast extract agar
      • Results after 3–5 days
  • PCR: high sensitivity, high specificity
  • Serology: : A four-fold rise in antibody titer confirms legionellosis. However, the antibody titers have low specificity and sensitivity, and seroconversion can take up to 12 weeks. Therefore, more rapid tests like the urinary antigen test or PCR are used more often.


  • Chest x-ray: usually abnormal, diffuse, interstitial infiltrates



Legionnaires' disease should be treated early with antibiotics, especially because of its high mortality rate of 10%.

In any case of atypical pneumonia, antibiotic treatment needs to cover Legionella!



  • Legionellosis is a notifiable disease
  • Course of action when contaminated water sources are detected in medical facilities
    • Contaminated water systems should be disinfected
    • Use terminal tap water filters, especially for high-risk patients (e.g., immunocompromised or the elderly)

Legionellosis is not transmitted from person to person, therefore, isolation is unnecessary!